1. Field of the Invention
The invention relates to a piezoelectric pump which uses a piezoelectric actuator.
2. Description of Related Art
A conventional piezoelectric pump 200 is shown in FIGS. 11-12 which uses a piezoelectric actuator. FIG. 11 is a perspective view which shows one example of such a conventional piezoelectric pump. FIG. 12 is a longitudinal sectional view of the piezoelectric pump shown in FIG. 11.
Piezoelectric pump 200 is structured to have an upper pump chamber main body 222, lower pump chamber main body 224, two piezoelectric bimorph elements 220A, 220B, inlet valve 226 and discharge valve 228. The pump chamber 254, enclosed by upper pump chamber main body 222, lower pump chamber main body 224, the two piezoelectric bimorph elements 220A, 220B, inlet valve 226 and discharge valve 228, is installed in piezoelectric pump 200.
The piezoelectric bimorph element 220A provides the upper surface of piezoelectric pump 200 and piezoelectric bimorph element 220B provides the lower surface of piezoelectric pump 200.
Inlet opening 201 and discharge opening 202 are formed by upper pump chamber main body 222 and lower pump chamber main body 224, and are installed in both ends of the piezoelectric pump 200 in the longitudinal direction of FIG. 12.
At inlet opening 201, the fluid flows into piezoelectric pump 200. Inlet valve 226 is installed in the inlet opening 201. Inlet valve 226 does not limit the inflow of the fluid into the piezoelectric pump 200, but prevents the outflow, or reverse flow, of the fluid to out of piezoelectric pump 200.
At discharge opening 202, the fluid flows out of the piezoelectric pump 200. Discharge valve 228 is installed in the discharge opening 202. Discharge valve 228 does not affect the outflow of the fluid to the outside of the piezoelectric pump 200 but prevents the inflow of fluid into piezoelectric pump 200, that is, reverse flow.
The piezoelectric bimorph element 220A is constructed from two piezoelectric ceramic boards 230A, 230B, two electrodes 232A, 232B and an electrode 234A. The electrode 234A is between the two piezoelectric ceramic boards 230A, 230B. The electrode 234A and the two piezoelectric ceramic boards 230A, 230B are between the two electrodes 232A, 232B. The electrodes 232A, 232B and the electrode 234A are connected to a AC power supply (not shown). A voltage with the same polarity is applied to the electrodes 232A, 232B from the AC power supply and a voltage having an opposite polarity is applied to the electrode 234A from the AC power supply. The piezoelectric bimorph element 220A flexes vertically, as shown FIG. 12 by the dashed lines, upon application of the voltage from the AC power supply.
The piezoelectric bimorph element 220B is constructed from two piezoelectric ceramic boards 230C, 230D, two electrodes 232C, 232D and an electrode 234B. The electrode 234B is between the two piezoelectric ceramic boards 230C, 230D. The electrode 234A and two piezoelectric ceramic boards 230C, 230D are between the two electrodes 232C, 232D. The electrodes 232C, 232D and the electrode 234B are connected to a AC power supply (not shown). A voltage with the same polarity is applied to the electrodes 232C, 232D from the AC power supply and a voltage having an opposite polarity is applied to the electrode 234B from the AC power supply. The piezoelectric bimorph element 220B also flexes vertically, as shown in FIG. 12 by the dashed lines, upon application of the voltage from the AC power supply.
The movement of the piezoelectric bimorph element 220A only is explained because both the piezoelectric bimorph element 220A and the piezoelectric bimorph element 220B operate in the similar manner.
The piezoelectric bimorph element 220A is polarized, as shown FIG. 12 by the solid line arrows. A voltage of a positive polarity is applied to the electrodes 232A, 232B from the AC power supply and a voltage with a negative polarity is applied to the electrode 234A from the AC power supply. An electric field, which is in the reverse direction of the poling direction, is produced in upper piezoelectric ceramic board 230A and an electric field in the same direction as the poling direction is produced in the lower piezoelectric ceramic board 230B. According to the characteristics of the piezoelectric ceramic board, the upper piezoelectric ceramic board 230A extends in a horizontal direction and the lower piezoelectric ceramic board 230B shrinks in the horizontal direction as shown in FIG. 12. As a result, the piezoelectric bimorph element 220A flexes as shown in the upper dashed lines of FIG. 12.
When a voltage of a negative polarity is applied to the electrodes 232A, 232B from the AC power supply and a voltage having a positive polarity is applied to the electrode 234A from the AC power supply, then, an electric field in the same direction as the poling direction is produced in the upper piezoelectric ceramic board 230A and an electric field in the reverse direction of the poling direction is produced in the lower piezoelectric ceramic board 230B. As a result, the upper piezoelectric ceramic board 230A shrinks in a horizontal direction and the lower piezoelectric ceramic board 230B extends in the horizontal direction, as shown in FIG. 12. As a result, the piezoelectric bimorph element 220A flexes as shown in the lower dashed lines of FIG. 12.
For this piezoelectric pump, the lower piezoelectric bimorph element 220B is controlled to flex in the direction of the lower dashed lines when the upper piezoelectric bimorph element 220A is controlled to flex in the direction of the upper dashed lines (FIG. 12). Moreover, the lower piezoelectric bimorph element 220B is controlled to flex in the direction of the upper dashed lines when the upper piezoelectric bimorph element 220A is controlled to flex in the direction of the lower dashed lines (FIG. 12). The volume of the pump chamber 254 increases and decreases alternately by the flexing movement.
When the volume of the pump chamber 254 increases, a negative pressure is applied to the fluid in the pump chamber 254 and fluid from outside of the piezoelectric pump 200 is drawn from the inlet opening 201 into the pump chamber 254 through inlet valve 226. When the volume of the pump chamber 254 decreases, a positive pressure is applied to the fluid in the pump chamber 254 and the fluid is discharged from the discharge opening 202 through the discharge valve 228 to outside of the piezoelectric pump 200.
However, for the above described piezoelectric pump, at least one piezoelectric bimorph element, or more, is necessary for each pump chamber. Therefore, there is a problem in that the structure becomes complex because of an increased number of parts. In addition, the manufacturing costs rise when a piezoelectric pump having a number of discharge openings is constructed. Moreover, there is a problem that miniaturization of the piezoelectric pump is very difficult because of the increased number of parts.